Plasma spraying method and apparatus

ABSTRACT

Disclosed herein is a plasma spraying apparatus comprising a plasma gun with a power reactor which includes a pair of spaced electrodes connected to a source of voltage, and winding means for controlling current flow between the electrodes, together with a control circuit connected to the winding means for maintaining a uniform power level between the electrodes regardless of variance in voltage and current flow which could occur between electrodes in the absence of the control circuit.

United States Patent Waskiewicz et a1.

1 51 Sept. 30, 1975 3.388.318 6/1968 O'Brien 1 3.487.291 12/1969 Dowgiallo, Jr. 3.517.301 0/1970 Huber 323/211 3.527.022 9/1970 Archer et a1 315/111 X Primary E.\'un1im'r-Paul L. Genslcr Arturnqr. Agent, or FirmMichael. Best & Friedrich ABSTRACT Disclosed herein is a plasma spraying apparatus Com prising a plasma gun with a power reactor which includes a pair of spaced electrodes connected to a source of voltage. and winding means for controlling current flow between the electrodes. together with a control circuit connected to the winding means for maintaining a uniform power level between the electrodes regardless of variance in voltage and current flow which could occur between electrodes in the absence of the control circuit.

11 Claims, 1 Drawing Figure 85 own/.44

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US. Patent Sept. 30,1975

PLASMA SPRAYING METHOD AND APPARATUS BACKGROUND OF THE INVENTION The invention relates generally to plasma spraying apparatus and more broadly, to apparatus for controlling the power level between a pair of electrodes.

When using plasma guns or torches, variation in the power level applied between the electrodes causes highly undesirable variation in the resulting deposit. Such variation can occur due to electrode wear, variation in line voltage, variation in the gas employed in the gun or torch, and for other reasons.

SUMMARY OF THE INVENTION The invention provides apparatus for maintaining a constant power level between the electrodes of a plasma gun or torch. More specifically. plasma spraying apparatus commonly includes a winding which controls the current flow between the electrodes and which, in cooperation with the applied voltage, determines the power level employed between the electrodes. In accordance with the invention, there is provided a control circuit which is connected to the control winding and which varies the potential applied thereto so as to vary the current between the electrodes in such manner as to maintain a uniform power level.

More specifically, the invention provides a plasma spraying apparatus comprising a plasma gun including a pair of spaced electrodes connected to a source of voltage. and winding means for controlling current flow between the electrodes, together with a control circuit connected to the winding means for maintaining a uniform power level between the electrodes regardless of variance in the voltage or current flow which could run between the electrodes in the absence of the current flow controlling means.

Still more specifically, the control circuit comprises means for introducing a reference power level signal, means for generating a power level signal proportional to the power level between the electrodes, means for combining the reference power level signal and the power level signal to generate a power level differential signal and means for applying the power level differen tial signal to the winding means to vary the current flow between the electrodes so as thereby to maintain a uniform power level. The means for generating the power level signal comprises means for generating a voltage signal proportional to the voltage applied to the electrodes means for generating a current signal propor tional to the current flow between the electrodes. and means for multiplying the voltage signal and current signal to generate the power level signal.

Still more specifically in accordance with the invention, the control circuit includes means for introducing a manual power level signal, and the means for applying the power level differential signal to the winding means includes means for combining the power level differential signal and the manual power level signal to generate a power level control signal which is applied to the winding means.

In still further accordance with the invention, there is provided a switch operable between an automatic position and a manual operating position disconnecting the power level differential signal from the means for combining the power level differential signal and the manual power level signal, whereby said power level control signal is regulated solely by said manual power level signal.

In still further accordance with the invention, the circuit further includes means for suppressing current flow between the electrodes during the initiation of plasma gun operation. The current flow suppressing means includes a time-delay relay operable to connect the current suppressing means in the circuit for a given time, together with means for switching the circuit between an automatic operating mode and a manual operating mode. Still further the current suppressing means includes the current signal generating means together with means for introducing a reference current signal, and means for combining the current signal and the reference current signal to generate a differential current signal which is applied through the time-delay relay during operation of the circuit in the automatic mode.

The invention also provides a method of maintaining a uniform power level between a pair of spaced electrodes connected to a transformer including a coil controlling current flow between the electrodes, said method comprising the steps of introducing a reference power level signal, generating a power level signal proportional to the power level between the electrodes, combining the reference power level signal and the power level signal to generate a power level differential signal, and applying the power level differential signal to the coil controlling current flow between the electrodes to vary the current flow between the electrodes so as to maintain a uniform power level.

One of the principal features of the invention is the provision of the control circuit for maintaining a uniform power level across the electrodes of a plasma torch or gun, notwithstanding variation in voltage or current which can occur between the electrodes in the absence of the control circuit. Another principal feature of the invention is the provision of means for suppressing high current conditions during the initiation of plasma spraying operations so as to avoid rapid deterioration of the electrodes.

Other features and advantages of the invention will become known by reference to the following drawing, general description and claims.

THE DRAWING The drawing is a fragmentary schematic view of a plasma spraying apparatus including a power level control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purposes of description and should not be regarded as limiting.

GENERAL DESCRIPTION Shown schematically in the drawing is a plasma spraying apparatus 11 which comprises a conventional plasma gun or torch 13 including a pair of spaced electrodes 15. The spraying apparatus also includes a reactor 16 including means for controlling current flow between the electrodes 15. In the illustrated construction, such current flow controlling means comprises a reactor control or biasing winding 21 forming a part of a transformer 23 which also includes a reactor coil 25 connected in series with the pair of electrodes and to a rectified alternating line voltage source 27.

Also included in the apparatus 11 is a control circuit 31 for maintaining a uniform power or wattage level between the electrodes 15 regardless of variation in the voltage between the electrodes 15 or the current flow which can occur between the electrodes 15 in the absence of the control current 31. More specifically, the control circuit is connected to the reactor control winding 21 and serves to control the potential applied to the control winding 21 and thereby to control the current flow between the electrodes 15.

Still more specifically, the control circuit 31 is selectively operable in an automatic mode and in a manual mode and includes means 33 for introducing a reference power level signal which is of adjustably constant potential and which is of one polarity, either positive or negative. The control circuit also includes means 35 for generating a power level signal proportional to the power level between the electrodes 15, which signal is of opposite polarity from the reference power level signal and which varies in accordance with the power level between the electrodes 15. In addition, the control circuit 31 includes means 37 for combining the reference power level signal and the power level signal to generate a differential signal of variable potential and polarity, and means 39 for applying the differential signal to the reactor current flow control means to vary the How between the electrodes 15 so as to maintain a uniform power level.

In further accordance with the invention, the means 35 for generating the power level signal comprises means 41 for generating a voltage signal proportional to the line voltage applied to the electrodes 15 and of a polarity opposite from the reference power level signal, together with means 43 for generating a current signal proportional to the current flow between the electrodes 15 and of a polarity opposite from the reference power level signal; and means 45 for multiplying the voltage signal and the current signal to generate the power level signal.

Still more particularly in accordance with the invention, the control circuit 31 also includes means 47 for introducing or applying a manual control or power level signal which is of adjustably constant potential and which is of one polarity, either positive or negative, and means 49 for combining the manual power level signal with the power level differential signal to generate a power level control signal which is applied to a pulse width modulator 51 which, in turn, controls the potential applied to the control or bias winding 2] in accordance with the power level control signal.

Any suitable plasma gun or torch 13 including a pair of electrodes 15 can be employed. The reactor 16 comprises a transformer including the control or bias winding 21 and the reactor power winding or coil 25 which is connected in series with the pair of electrodes 15.

More specifically, in the construction shown in the drawings, the means 33 for introducing a reference power level signal comprising an adjustable potentiometer 55 connected between a source of current 57 and common terminal or ground 59, and including an adjustable wiper 61. In turn, the wiper 61 is connected by a lead to one input terminal 63 of a voltage follower 65 in the form of an integrated circuit amplifier. The other input terminal 67 of the voltage follower 65 is connected by a shunt lead 69 to the output terminal 71 of the voltage follower 65. In turn, the output terminal 71 is connected by a lead 73 through a resistor 75 to one input terminal 77 of a differential detector 79 which forms a part of the means 37 for combining the reference power level signal and the power level signal and which is in the form of an integrated circuit operational amplifier.

The power level signal generating means 35, as previously indicated, comprises means 41 for generating a voltage signal proportional to the line voltage applied to the electrodes 15, means 43 for generating a current signal proportional to the current flow between the electrodes 15, and means 45 for multiplying the voltage signal and the current signal to generate the power level signal.

The voltage signal generating means 41 comprises a volt meter 81 which reads the voltage applied to the electrodes 15 and which supplies a signal proportional to the applied voltage. The volt meter 81 is connected by a lead to one input terminal 83 of a filter 85 in the form of an integrated circuit operational amplifier. The other input terminal 87 of the filter 85 is connected by a lead to ground or common 59. The output terminal 91 of the filter 85 passes the voltage signal through a lead to one input terminal 93 of the multiplier 45 which functions to multiply the voltage signal and the current signal and thereby to generate a power level signal proportional to the power level between the electrodes.

The current signal generating means 43 comprises a current meter 95 which is in the form of a device measuring voltage drop across a shunt lead and which supplies a signal proportional to the current flow between the electrodes 15. The current meter 95 is connected by a lead to one input terminal 97 of a filter 99 in the form of an integrated circuit operational amplifier. The other input terminal 10] of the filter 99 is connected to ground or common 59. The output terminal 103 of the filter 99 passes the current signal through a lead 104 to a second input terminal 105 of the multiplier 45.

The output of the multiplier 45 i.e., the power level signal, is fed from the multiplier output terminal 107 through a lead 109 and resistor 111 to the previously mentioned input terminal 77 of the differential power level detector 79.

The previously mentioned input terminal 77 of the power level differential detector 79 is also connected through a shunt lead 113 including a resistor 115 to the output terminal 117 of the differential detector. The other input terminal 1 19 of the differential detector 79 is connected to the common terminal or ground 59.

The output terminal of the amplifier 127 is also connected through a lead 146 including a resistor 148 to a null meter 150 which is connected to the common terminal or ground 59. Included in the null meter is a high limit relay switch which is hereinafter further referred to, which is normally open, and which closes when the potential measured by the meter reaches a predetermined high level. The null meter indicates both the polarity and potential of the power level dif ferential signal as it leaves the amplifier.

The output of the power level differential detector 79 is the power level differential signal which is fed from the differential detector output terminal 1 17 through a lead 121 including a resistor 123 to one input terminal 125 of an amplifier 127 in the form of an integrated circuit operational amplifier. The other input terminal 129 of the amplifier 127 is connected to the common terminal or ground 59. A shunt lead 131 including a resistor 133 is connected between the input terminal 125 and an output terminal 135 of the amplifier 127, which output terminal 135 is connected by a lead 137 including two series connected resistors 139 and 141 to one input terminal 143 of a summing amplifier 145 in the form of an integrated circuit operational amplifier.

The output terminal 117 of the power level differential detector 79 is also connected through a lead 147 including two series connected resistors 149 and 151 to one input terminal 153 of an integrator 155 in the form of an integrated circuit operational amplifier. The other input terminal 157 of the integrator 155 is connected through a lead to ground or common 59 and to a wiper 159 of a variable resistor or potentiometer 161 which has one end or terminal 163 electrically unconnected and an other end or terminal 165 connected between the resistor 149 and 161. The variable resistor 161 controls the speed of integration.

The first mentioned input terminal 153 of the integrator 155 is also connected to a lead 167 which includes a capacitor 169 and which is connected to the output terminal 171 of the integrator 155. Connected between the input and output terminals 153 and 171 of the integrator 155 and in parallel with the capacitor 169, is a series connected resistor 173 and the previously mentioned high limit relay switch 179 which is operated by the null meter 150, which is normally open, and which when closed, serves to effectively remove the integrator 155 from the circuit 31.

The output of the integrator 155 is an integrated power level differential signal which is transmitted through a lead 181, including two series connected resistors 183 and 185, to the input terminal 143 of the summing amplifier 145. In order to limit the integrated power level differential signal to a given or predetermined level there is connected to the lead 181, between the resistors 183 and 185, a pair of oppositely arranged diodes 187 and 189 which are connected to common or ground 59 and which pass to ground all current at a potential above the voltage drop across the diodes 187 and 189.

The input terminal 143 of the summing amplifier 145 is also connected by a lead 191 including a resistance 193 to an output terminal 195 ofa voltage follower 197 which in the form of an integrated operational amplifier and which forms part of the means 47 for applying an introducing a manual power level signal. One input terminal 199 of the voltage follower 197 is connected to the wiper of a variable resistor or potentiometer 203 which is connected between a suitable source of current 205 and the common terminal or ground 59 and which functions to introduce the manual power level signal. The other input terminal 207 of the voltage follower 197 is connected through a shunt lead 209 to the before mentioned output terminal 195. The potentiometer 203 can accordingly be used to regulate the control winding 21 in the reactor when the control circuit 31 is in the manual operation mode and to affect control winding regulation when the control circuit 31 is in the automatic operation mode.

The summing amplifier also includes another input terminal 211 which is connected to the common terminal or ground 59 and further includes an output terminal 217 which is connected to the input terminal 143 through a shunt lead 213 including a resistor 215. The power level control signal passes from the summing amplifier 145 through the output terminal 217 which is connected through a lead 219 to the pulse width modulator 51 which, as already indicated, controls the voltage applied to the control winding 21 in accordance with the incoming power level control signal. More specifically, the pulse width modulator regulates current flow through a lead 220 from a current source 221 and through a lead 223 to the control winding 21 which, in turn, is connected through a lead 225 to the common terminal or ground 59.

As already mentioned, the control circuit 31 can be operated either in automatic or manual. In this regard, there is incorporated in the circuit 31 a pair of simultaneously operated double throw switches 231 and 233. The switch 231 includes a switch member movable between automatic and manual terminals 237 and 239 respectively, and, when engaged with the manual terminal 239, is connected in parallel with the high limit relay 179. Accordingly, when the circuit 31 is in manual, the integrator is effectively removed from the circuit 31 in the same manner as when the high limit relay switch 179 is closed. The automatic terminal 237 of the switch is electrically unconnected.

The other switch 233 includes a switch member 241 movable between automatic and manual terminals 243 and 245, respectively, and when engaged with the automatic terminal 243, connects the resistors 139 and 141. However, when the switch member is moved to the manual terminal 245, the resistors 139 and 141 are disconnected to discontinue application of the amplified power level differential signal to the summing amplifier 145. Under such circumstances, the power level control signal is regulated solely by the manual power level signal.

In further accordance with the invention, there is provided means for preventing abnormally high current between the electrodes 15 upon initial operation of the plasma torch 13 when the circuit 31 is in the automatic mode. More specifically, the current signal from the output terminal 103 of the filter 99 is also applied through a lead 247 including a resistor 249 to one input terminal 251 of a current error detector 253 in the form of an integrated circuit operational amplifier. Connected to the other input terminal 254 of the current error detector 253 is a lead 255 which is connected to common or ground 59, which is also connected to one end or terminal 257 of a potentiometer 259 which, at its other end 260, is electrically unconnected and which includes a wiper 263. The lead 255 also is connected through a resistor 261 to the potentiometer wiper 263 and to a second resistor 265 which in turn is connected to the input terminal 251. Thus, adjustment of the wiper 263 will provide a reference current signal which is of adjustably constant potential and which is of a plurality opposite from the polarity of the current signal.

Also connected to the input terminal 251 is a shunt lead 261 which includes a resistor 263 and which is also connected to the output terminal 265 of the detector 253. The current signal differential detector produces a current differential signal which is applied from the output terminal 265 through a lead 267 including a resistor 269 to one terminal of a time-delay switch 273. The lead 267 is also connected to the anode of a diode 275 having a cathode connected to common or ground 59 and to the manual terminal 245 of the switch 233.

The time-delay switch 273 includes a switch member or blade 279 which is connected to the automatic terminal 243 of the switch 233 and which is movable between the terminal 271 and a terminal 281 connected to the resistor 139. Thus the resistors 139 and 141 are connectable and disconnectable relative to each other through the series arranged switches 233 and 273.

Upon initiation of plasma torch operation, the switch member 279 is connected to the terminal 271 so as thereby to supply the current differential signal to the summing amplifier 145 and to disconnect the summing amplifier 145 from the amplified power level differential signal. However, after the expiration of a short period of time, the switch member 279 disconnects from the terminal 271 and engages the terminal 281 so as thereby to connect the summing amplifier 145 to the amplifier power level differential signal and to disconnect the current differential signal from the summing amplifier 145.

In the control circuit 31, the integrator 155, in effect, gives the equivalent of high loop gain with no dangerous reactions, such as noticeable hunting or oscillatory reactions which could result in possible failure of the plasma gun or its power supply. The output voltage of the integrator 155 keeps changing until the power level differential signal reaches Zero. The integrator 155 maintains this output voltage, thereby holding the power level or wattage between the electrodes 15. Any new power level differential signal will again change the output voltage of the integrator 155 to maintain the power level determined by the reference power level signal. In addition, a conventional voltage feedback loop with a gain low enough to cause minimal oscilla tions is used in parallel with the integrator feedback loop. This helps to make up for the slower response of the integrator 155.

Various aspects of the invention are set forth in the following claims:

What is claimed is:

1. Plasma spraying apparatus comprising a plasma gun including a pair of spaced electrodes connected to a source of voltage and operable to generate a plasma in response to current flow between said electrodes, a transformer having a first winding electrically connected in series between one of said electrodes and said voltage source, said transformer also including a second winding, and a control circuit for maintaining a uniform power level between said electrodes regardless of variance in voltage and current flow which can occur between said electrodes in the absence of said control circuit, said control circuit including said second winding.

2. Plasma spraying apparatus in accordance with claim I wherein said control circuit comprises means for introducing a reference power level signal, means for generating a power level signal proportional to the power level between said electrodes, means for combining the reference power level signal and the power level signal to generate a power level differential signal, and means for applying said power level differential signal to said current flow controlling means to vary the current flow between said electrodes so as to maintain a uniform power level.

3. Plasma spraying apparatus in accordance with claim 2 wherein said means for generating said power level signal comprises means for generating a voltage signal proportional to the voltage applied to said electrodes, means for generating a current signal proportional to the current flow between said electrodes. and means for multiplying said voltage signal and said our rent signal to generate said power level signals.

4. Apparatus in accordance with claim 2 wherein said control circuit further includes means for introducing a manual power level signal, and said means for applying said power level differential signal to said current flow controlling means includes means for combining said power level differential signal and said manual power level signal to generate a power level control signal which is applied to said current flow controlling means.

5. Plasma spraying apparatus in accordance with claim 4 wherein said control circuit includes a switch operable between an automatic position and a manual position disconnecting said power level differential sig nal from said means for combining said power level differential signal and said manual power level signal, whereby said power level control signal is regulated solely by said manual power level signal,

6. Plasma spraying apparatus in accordance with claim 4 wherein said circuit further includes an amplifier connected between said power level differential signal generating means and said power level control signal generating means, an integrator connected between said power level differential signal generating means and said power level control signal generating means in parallel with said amplifier, and means for shunting said integrator in the event said power level differential signal is above a predetermined level.

7. Plasma spraying apparatus in accordance with claim 6 wherein said circuit further includes means connected to said integrator for passing to ground current above a predetermined potential.

8. Plasma spraying apparatus comprising a plasma gun including a pair of spaced electrodes connected to a source of voltage and operable to generate a plasma in response to current flow between said electrodes, means electrically connected in series between one of said electrodes and said voltage source for controlling current flow between said electrodes, a first control circuit means connected to said current controlling means for maintaining a uniform power level between said electrodes regardless of variance in voltage and current flow which can occur between said electrodes in the absence of said first control circuit means, a second circuit means for suppressing current between said electrodes during initiation of plasma generation, and switch means for detachably connecting said second circuit means to said first circuit means for effecting current suppression during initiation of plasma generation.

9. Plasma spraying apparatus in accordance with claim 8 wherein said current suppressing means includes said current signal generating means, means for introducing a reference current signal, and means for combining said current signal and said reference current signal to generate a differential current signal for switching said circuit between an automatic operating mode and a manual operating mode and wherein said time-delay switch is operable to electrically connect said current suppressing means in said circuit when said circuit is in said automatic operating mode. 

1. Plasma spraying apparatus comprising a plasma gun including a pair of spaced electrodes connected to a source of voltage and operable to generate a plasma in response to current flow between said electrodes, a transformer haVing a first winding electrically connected in series between one of said electrodes and said voltage source, said transformer also including a second winding, and a control circuit for maintaining a uniform power level between said electrodes regardless of variance in voltage and current flow which can occur between said electrodes in the absence of said control circuit, said control circuit including said second winding.
 2. Plasma spraying apparatus in accordance with claim 1 wherein said control circuit comprises means for introducing a reference power level signal, means for generating a power level signal proportional to the power level between said electrodes, means for combining the reference power level signal and the power level signal to generate a power level differential signal, and means for applying said power level differential signal to said current flow controlling means to vary the current flow between said electrodes so as to maintain a uniform power level.
 3. Plasma spraying apparatus in accordance with claim 2 wherein said means for generating said power level signal comprises means for generating a voltage signal proportional to the voltage applied to said electrodes, means for generating a current signal proportional to the current flow between said electrodes, and means for multiplying said voltage signal and said current signal to generate said power level signals.
 4. Apparatus in accordance with claim 2 wherein said control circuit further includes means for introducing a manual power level signal, and said means for applying said power level differential signal to said current flow controlling means includes means for combining said power level differential signal and said manual power level signal to generate a power level control signal which is applied to said current flow controlling means.
 5. Plasma spraying apparatus in accordance with claim 4 wherein said control circuit includes a switch operable between an automatic position and a manual position disconnecting said power level differential signal from said means for combining said power level differential signal and said manual power level signal, whereby said power level control signal is regulated solely by said manual power level signal.
 6. Plasma spraying apparatus in accordance with claim 4 wherein said circuit further includes an amplifier connected between said power level differential signal generating means and said power level control signal generating means, an integrator connected between said power level differential signal generating means and said power level control signal generating means in parallel with said amplifier, and means for shunting said integrator in the event said power level differential signal is above a predetermined level.
 7. Plasma spraying apparatus in accordance with claim 6 wherein said circuit further includes means connected to said integrator for passing to ground current above a predetermined potential.
 8. Plasma spraying apparatus comprising a plasma gun including a pair of spaced electrodes connected to a source of voltage and operable to generate a plasma in response to current flow between said electrodes, means electrically connected in series between one of said electrodes and said voltage source for controlling current flow between said electrodes, a first control circuit means connected to said current controlling means for maintaining a uniform power level between said electrodes regardless of variance in voltage and current flow which can occur between said electrodes in the absence of said first control circuit means, a second circuit means for suppressing current between said electrodes during initiation of plasma generation, and switch means for detachably connecting said second circuit means to said first circuit means for effecting current suppression during initiation of plasma generation.
 9. Plasma spraying apparatus in accordance with claim 8 wherein said current suppressing means includes saiD current signal generating means, means for introducing a reference current signal, and means for combining said current signal and said reference current signal to generate a differential current signal.
 10. Plasma spraying apparatus in accordance with claim 8 wherein said current suppressing means includes a time-delay switch operably connecting said current suppressing means in said control circuit for a given time.
 11. Plasma spraying apparatus in accordance with claim 10 wherein said circuit further includes means for switching said circuit between an automatic operating mode and a manual operating mode and wherein said time-delay switch is operable to electrically connect said current suppressing means in said circuit when said circuit is in said automatic operating mode. 